(Not Applicable)
When rechargeable storage batteries are used in electric systems, the requirements for power to be delivered to the connected loads in discharge and/or the availability of power for charging typically do not have values that allow maximization of life of the batteries and maximization of the performance of the systems of which the batteries are a part.
FIG. 1 shows a typical prior art electrical system which provides uninterrupted electrical power to a load or loads 60. The system includes a primary power source such as AC source 10 (for example, a stand-alone AC generator or generators, or other source of AC electricity such as an electric utility), a single-throw switch 15 which allows the AC source 10 to be disconnected from the system, double-throw switches 22 and 24 to alternately connect an AC to DC converter 20 or a DC to AC converter 26 between the AC source or load(s) and battery 1. The converters 20 and 26 with the switches 22 and 24 may optionally be a two-way converter which combines the function of 20 and 26 into a single unit. The battery 1 is shown as consisting of two strings, String A and String B, but may optionally consist of any number of strings equal or greater than 1.
The generic electric system shown in FIG. 1 may, but not necessarily, also include supplemental generators, such as that shown as DC generator 12, connected to the battery via power conversion equipment such as the DC to DC converter 14. In one typical application, DC generator 12 is a photovoltaic array for generating DC from solar power.
In FIG. 1, the electric system is shown with the switches 15, 22 and 24 in a position such that the battery 1 can be charged and the load(s) 60 can be supplied with electric power from the AC source 10. The direction of current flow in the various legs of the circuit is indicated with the symbol {circle around (xe2x86x92)} in FIG. 1 and in subsequent figures. If electricity is not being supplied by the AC source 10 for any reason, switch 15 is opened, and switches 22 and 24 are put into their alternate position so that the load(s) 60 can be supplied with electricity from the battery 1 via the DC to AC converter 26. Alternatively, if the DC to AC converter 26 and the AC source 10 are synchronized in a manner conventional in the art, then switch 15 can remain closed. In this event, discharge of the battery can supplement the supply from the AC source so that the battery 1 and the AC source 10 jointly supply the load(s) 60.
The electrical power requirements of the load(s) and the capabilities of the AC source in electric systems are often such that the battery cannot be charged and/or discharged in the manner required to maximize both the life of the batteries and the performance of the electric system.
For example, batteries based on the zinc/bromine chemistry need to be completely discharged occasionally to maximize their utility. However, such batteries should never be completely discharged when they are used to provide back-up power for critical manufacturing processes, as they otherwise might not be available at critical times.
As another example, certain types of nickel/cadmium batteries exhibit a memory effect which results in an apparent loss of available capacity when repeatedly partially discharged and then recharged. This loss of capacity can be recovered by completely discharging and recharging the battery. These batteries are sometimes used in hybrid electric vehicles where power for recharge is only available during vehicle operation, so the frequent complete discharges these batteries require for optimal life and performance cannot be effected.
As a third example, the state of charge of lead-acid batteries used to help match supply from an electric generator (or electric generators if they are connected into an electricity supply network) and the demand (load requirements) from customers connected to that generator, cannot be optimally managed because the power available for recharge or that required in discharge are determined by the difference between the supply available from the generator(s) and the load demanded by customers. Lead-acid batteries perform best and live longest if each charge is completed (finished) properly and if they are not discharged too deeply. On the other hand, system performance will be maximized if the generator is used only when absolutely necessary. None of these optimization criteria can be strictly adhered to because of the highly variable power available or required in the supply-load matching process. Finishing charge and avoiding overdischarge of lead-acid batteries, and optimizing the performance of systems using lead-acid batteries to help generators match supply and demand, are one of the most important potential applications for the invention disclosed herein.
Most battery manufacturers offer guidelines for ways to optimally charge and discharge their products so as to maximize life and performance. Implementation of these guidelines is made complicated for users by virtue of the fact that most batteries are in fact a collection of individually manufactured units, each of which has slightly different performance characteristics.
The most fundamental unit of batteries is a cell, a unit of 1 to 4 volts depending on the chemistry on which the battery is based. A cell consists of a collection of positive electrodes in parallel and negative electrodes in parallel, juxtaposed so to provide the power and the ampere-hour capacity specified. Sometimes, a few cells (of the order of three to eight) are assembled into modules, with the series electrical connections between the cells being effected during manufacture. Cells or modules are then electrically connected in series at the point of use to form strings.
Other batteries are based on cell-stacks consisting of a number of cells electrically connected in series. In this case, modules are fabricated by connecting a number of cell-stacks in electric series and/or parallel. With some battery chemistries, battery auxiliaries, such as pumps for flow batteries or thermal management hardware for both conventional and advanced batteries, are incorporated with the cells or the cell-stacks into a module.
A storage battery (sometimes referred to as a battery system) consists of a number of cells or modules arranged in series and/or parallel arrays. Cells or modules connected in series are collectively referred to as a battery string. Battery strings may then be electrically paralleled. Occasionally, there is only one module in a string, and infrequently, cells are placed only in parallel in a battery. In these cases, there is no meaning to the term string, but if there were, a string would consist of one module or one cell. Alternatively, a battery consists at times of only one string of cells or modules. As discussed here and after, the current invention does not relate to such single string batteries; at least two strings in parallel are required for operation of this invention, although each of the strings may consist of one or more cells or modules in series.
The number of cells or modules in series in a string or battery is determined by the voltage desired for the battery system, which is in turn set by the requirements of the charging and discharging equipment to which the battery is connected. The charging and discharging equipment is generally referred to as power conversion equipment. The number of strings in parallel is determined by the capacity, i.e., the number of ampere-hours, or the energy rating, i.e., the number of watt-hours, that is desired by the user of the battery system.
In order to standardize the terms used herein, FIG. 2 shows a charge profile for a lead-acid battery. When a constant current is initially applied to a discharged battery, bulk charging occurs and the voltage across the battery increases in a generally exponential manner to the voltage set-point level of the battery. At this point, the charging voltage is held constant and the current begins to fall off in a generally exponential manner. This finish charging period continues until the current reduces asymptotically to a relatively low value. Finish charging is ended by a time limit, low current threshold, or amp-hour overcharge level. A nickel/cadmium battery has a somewhat similar profile, but a zinc/bromine battery is normally charged with a fixed, constant power until charging is deemed complete.
While the term xe2x80x98finish chargingxe2x80x99 is well-understood by both battery manufacturers and users, xe2x80x98equalizationxe2x80x99 is not. In fact some authors incorrectly use the terms interchangeably, and in other writings there is some confusion over the terms. Here, finish charging is defined as a process at the end of nearly every bulk charge when the battery has reached regulation voltage and the charge current tapers (i.e., reduces in magnitude) because of an exponential increase in the effective resistance of the cells of the battery as further charge is applied. Finish charging a lead-acid battery typically takes on the order of one to four hours.
xe2x80x98Equalizationxe2x80x99 has two distinct definitions in the battery field. For one definition, which is not utilized in this invention, it means actively adjusting the charge of individual cells in a string in order to restore each cell to an equal state of charge. For the other definition, which is utilized herein, equalization in a lead-acid battery is accomplished by an extended-period, relatively low current, charge following a regular recharge. During equalization, the voltage is raised a little above the fully-charged open circuit voltage and current is limited for a period of on the order of twelve to twenty-four hours. The current drops during the early part of the equalization process and for most of the process is typically a few percent of the normal charging current. Thus, the electrical power required for equalization is a small fraction of the power normally required for charging.
Finish charging and equalization of lead-acid batteries, and the complete discharge of a nickel/cadmium or zinc/bromine battery are all examples of battery management procedures, as defined in this invention, that, if properly done, will help maximize the life of the battery, but are not necessary for satisfactory short-term operation.
A specific example of an electric system for which the current invention may find utility is a solar hybrid system. Solar hybrid systems are increasingly used to provide power to electricity end-users at locations that are remote from the transmission and distribution systems of utilities. The design of a solar hybrid system is much like the generic electric system shown in FIG. 1, with the DC generator 12 being a solar photovoltaic (PV) array and AC Source 10 being a fossil-fueled engine-generator. A controller 70 senses the system requirements and capabilities and controls the components in a manner well known in the art.
Lead-acid batteries are frequently used in solar hybrid systems. Each battery system consists of a number of 2 volt cells, or 6 volt (three-cell) or 12 volt (six-cell) modules, connected in series to form a string having a useful output voltage. Most solar hybrid systems use a plurality of cells or modules in a series string to provide a sufficient output voltage. In order that the battery system has adequate capacity to cover relatively long periods without solar energy and without having to turn on the generator (for example: long winter nights) most solar hybrid systems use a plurality of strings in parallel.
The operation of a typical solar hybrid system is as follows: on a sunny day, direct current (DC) from the photovoltaic array 12 (the PV) is provided to the power conversion equipment 14 and 26, which may convert it to a different DC value and then to alternating current (AC) to power the load, or the excess energy at the different DC value may be used to charge the battery. If there is not enough solar energy to generate sufficient output from the PV, or at night when the sun is down, the load 60 is supplied with energy from the battery 1. When the battery 1 needs charging, the generator 10 supplies the load and recharges the battery 1. The generator 10 is turned off when the battery 1 is filly charged. From an operating maintenance cost standpoint, energy from the PV 12 is least expensive; supplying energy to the load from the storage battery I is more expensive because of the inefficiency of the battery and because use degrades the life of the cells; and operation by generator 10 is most expensive since a suitable high-grade fuel must be provided and the generator requires periodic maintenance. Furthermore, utilization of a generator at a small fraction of its power capability is particularly expensive since a generator is often inefficient under this operating condition and requires more maintenance.
During discharge mode, if solar energy is not available, the battery 1 (Strings A and B) provides the power for the user""s electrical loads 60 via power conversion equipment 26 that converts DC to AC. If solar energy is available, the PV 12 may provide some or all of the power for the user""s loads, and at times, the PV may be providing more power than needed by the customer, so that the battery becomes partially recharged even though in the discharge mode. When the battery voltage reaches a preset lower level, as measured by the power conversion equipment, the generator 10 is started so that the battery can be recharged. However, since all the strings of the battery are connected in parallel in current systems, as illustrated in FIG. 1, there is only one battery voltage that can be measured. Moreover, the battery voltage depends on the power that is being withdrawn from the battery, and the average discharge power is typically quite low in solar hybrid systems. Thus, the threshold for charge to be initiated may be reached during periods of high power demand. The consequence of these factors means that cells and modules within the battery can be discharged to a much greater extent than intended, with a resulting deleterious effect on battery life.
For much of the charge time with the AC source (the generator) 10, i.e., the bulk charging period, the lead-acid battery of this example (as with batteries based on other chemistries) can accept charge efficiently at almost any power level that can be provided by the source of charging energy. However, towards the end of charge, the effective resistance of the battery (defined as the ratio of the excess voltage required to pass the charging current to the charging current) increases and the efficiency of recharge (the fraction of the current being applied that increases the real state of charge of cells) decreases. The power conversion equipment 20 (acting as an AC to DC converter during battery charge) is set to reduce the charging current near the end of charge so that the charging inefficiency does not become too large. Charging is terminated when the charging current reduces to a preset lower limit, but it is not allowed to proceed for too long since the. generator is not efficient when the power is being produced below it""s rated value. As a result of this termination criteria, charging is usually not completed to a level recommended by the battery manufacturer, and the battery must be equalized periodically in order to ensure that capacity is not permanently lost. Any solar energy that is generated by the PV during finish charge or equalization by the generator cannot be used effectively and is lost, leading to further inefficiency. As a consequence of all these factors, more fuel is used by the generator than if the battery did not require finish charging and equalization, and the generator must be subjected to maintenance more frequently because it runs for a long period of time at low power. In addition, the equalization process itself and any failure to frequently complete finish charging both lead to a shorter life for the battery than would be expected under optimal charging conditions.
The deleterious effects of sub-optimal charging and discharging, as described above, are exacerbated by the fact that neither individual cells nor individual modules are identically constructed, so some cells and modules: 1) accept charge more efficiently during the finish charge; 2) discharge at higher voltage; as compared to other cells and modules; 3) some cells deteriorate at a faster rate than other cells. Since one bad cell may cause a battery to fail, it is desirable for efficient use of a battery system that the battery be charged and discharged in an optimal way.
The definition of equalization and finish charging frequently discussed in other patents is not the same as that used in the current document.
A more thorough explanation of the need for battery equalization may be found in U.S. Pat. No. 5,504,415 of Y. Podrazhansky et al., which patent discloses a system for equalizing individual batteries in a series string of batteries by shunting charging current around cells based on cell temperature. According to our definitions, the process of this patent would be called finish charging.
U.S. Pat. No. 5,905,360 of S. Ukita discloses an equalization system for a hybrid vehicle which uses a generator to equalize all modules in a series string, and then uses fully charged modules in the string to transfer charge to less fully charged modules. The load is not powered by the battery while this transfer is occurring, thus the battery system is not available to the load during the equalization procedure. Again, the process being accomplished here is what we would call finish charging.
Another system for equalizing a battery is shown in U.S. Pat. No. 6,150,795 of N. Kutkut et al, where battery charge equalization is carried out utilizing modules connectable in staggered relation between pairs of batteries in a series connected string of batteries. The device disclosed in this patent is commercially known as PowerCheq(trademark), a product of PowerDesigners, LLC of Middleton, Wis.
It is an object of this invention to provide a method for optimally charging and discharging multi-string batteries in electric systems so that battery life is maximized and the performance of the system of which the batteries form the storage component is also maximized.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, this invention is a method of optimally managing battery strings in an electric system for continuously powering the load, with at least two battery strings for selectively powering the load, and an electrical AC source for selectively powering the load and charging the battery strings. The method includes the processes of charging mulitple battery strings with the AC source; management of one battery string while the other battery strings power the load and without using the AC source; powering the load with all battery strings, without using the AC source, until discharge is deemed optimally complete; bulk charging all the battery strings using the AC source; and the sequential management of each battery string while the remaining battery strings power the load and without using the AC source. Alternately, the method includes the processes of charging multiple battery strings with the AC source; the sequential management of each battery string while the other battery strings remain available to power the load if necessary. As yet another alternative, the processes of charging multiple battery strings with the AC source; powering the load with all battery strings, without using the AC source, until discharge is deemed optimally complete; management of one battery string while the other battery strings are being charged using the AC source; completing the charge of all battery strings with the AC source; and then sequential managing each of the battery strings. Management may include finish charging, equalization, or total discharge, depending on the situation and battery type.